The primary purpose of this page is to inform prospective group members and collaborators of the type of work we are currently doing. While this list does not include all of our current work, it is a good overall description of the major research thrusts within our group. To make the connection between these projects and the graduate students who are working on them, go to our members link.

Much of our work is motivated and enhanced by the growing importance of nanomaterials and biomaterials. While we are often interested in direct characterization of materials on the nanometer scale, much of our experimental work probes 'microscopic' length scales that exist between the 'nanoscopic', molecular level and the bulk, macroscopic level. Our aim is to understand how molecular processes affect observable behavior at larger length scales. Our work falls into the general areas listed below. Overlap exists between some of these categories. For example, micromechanical techniques designed to study adhesive materials and polymer gels are also well suited for studying a range of biomaterials. Nevertheless, the following breakdown is a useful way of defining the core expertise of our laboratory.

How do chemical bonds between two surfaces manifest themselves in macroscopic adhesive forces? Often, these forces are much larger than what would be expected, based just on the strength of the bonds themselves. We have developed experimental methods for quantifying the underlying physics of adhesive interactions in 'soft' materials. Ref. 50 is a recent review that describes the underlying concepts in more detail. Materials of interest to us include common pressure sensitive adhesives used in 'sticky' tape, in addition to materials used in demanding applications such as electronic packaging and wound healing. The following list of examples is representative of our work in this area:

• In situ characterization of pressure sensitive adhesive films (ref. 32, ref. 40, ref. 48 and ref. 75).
• Viscoelastic properties of films made from mixtures of 'hard' and 'soft' particles (ref. 53).
• Deformation and debonding mechanisms of soft adhesive layers, including the role of geometrical confinement (ref. 47, ref. 61 and ref. 68).
• Adhesive transfer of a thin elastomeric film from an elastomeric substrate to a rigid surface (ref. 70, ref. 80).

In many cases we find that existing experimental methods are not suitable for obtaining the desired information, or that commonly employed methods contain additional information that is not conventionally extracted from the experimental data. We are involved in developing a series of techniques that are highly sensitive to interfacial structure and adhesion. Examples include the following:

• Development of a membrane contact technique for quantifying adhesion (ref. 92).
• Use of drop shape analysis to monitor monolayer formation at the oil/water interface.
• Use of the quartz crystal microbalance (QCM) as a contact sensor. We have published extensively in this area in recent years. Important examples of the most recent work are found in ref. 78, where we discuss geometrical effects that are important in a contact experiment where the crystal is not uniformly loaded, and ref. 86, where we describe the quantitative analysis for a general multilayer system, and apply it to a grafted polymer brush that is in contact with a polymeric membrane. An appropriately chosen membrane increases the base sensitivity of the quart crystal by a factor of 100, greatly enhancing the utility of the QCM as a simple tool for assessing the structure of interfacial layers in aqueous systems.

• The high-strain properties of alginate hydrogels (ref. 65). These naturally occurring materials are often used in biomedical applications because of their excellent mechanical toughness, and their ease of crosslinking by the addition of calcium ions.
• Thermoreversible acrylic triblock gels that form tough elastic gels at room temperature, but form low viscosity solutions when warmed to about 70 C. We have used these materials in a lot of our fundamental studies of adhesion and mechanical response because they have a very high mechanical strength, relative to the low-strain elastic modulus. Ref. 60 and ref. 89 summarize our understanding of the origins of the mechanical response of these unique materials. Click here for a brief description of the ordering process that gives rise to the thermoreversible elastic response in these materials.

The ability of these materials to rapidly transform from low-viscosity solutions to high-strength elastic gels has given rise to a materials processing application called thermoreversible gelcasting, described in more detail at this link. Ceramics processing applications have been developed in collaboration with the Faber group, and metals processing applications are being developed in collaboration with the Dunand group. Applications so far include the following:

• Development of a near-net-shape thermoreversible gelcasting technique for the formation of ceramic objects into complex shapes (ref. 56).
• Use of thermoreversible gelcasting to make ceramic laminates with a graded pore structure (ref. 59).
• Development of a titanium hydride route for casting bulk or porous titanium into complex shapes (in progress).

We are also developing a series of self-assembling hydrogels that rely on similar principles. These materials, which consist of acrylic triblock copolymers that produced gels in water by a solvent exchange process, are remarkably easy to form (see ref. 90). These materials are the focus of much of our ongoing work in the biomaterials area, as described in more detail below.

• Adhesion of hydrogels modified with the adhesive peptide, DOPA (dihydroxy phenylalanine). See ref. 64, ref. 82, and ref. 83 for examples of this collaboration with Phil Messersmith and his group here at Northwestern.
• Mechanical properties of tissue engineering scaffolds. Ref. 63 and ref. 73, are examples of our collaboration with the Shea group.
• Hydrogels with high toughness and low friction (in progress). The aim of this work is to understand the origins of the remarkable mechanical properties of cartilage, and to develop guidelines for the synthesis of cartilage replacement.
• Mechanical and chemical influences on the cell and tissue growth (in progress). The aim here is to use the triblock hydrogels as model systems with controlled moduli in the kilopascal range, and with chemical signals of interest incorporated into the midblock.

• Dynamics of metal particle nanoparticle diffusion at long times (ref. 38) and at times that are comparable to the relaxation time of the polymer (ref. 69). The short-time measurements utilize an x-ray standing wave technique that provides sub-nanometer resolution on the particle position. This work is done in collaboration with Dr. Jin Wang and others at Argonne National Laboratory. (Click here for more information.)
• Use of our self-assembling gels as a dispersing medium for carbon nanotubes (in progress). These systems are well-suited for fundamental investigations of elasticity in fibrous gels. This work is done in collaboration with the Brinson group.

Our work benefits enormously from collaborations with a variety of labs at Northwestern and throughout the world. Here's a list of some of our active collaborations:

• Tristan Baumberger and Christiane Caroli, CNRS-Université Paris 6 (Paris, France): Fracture of gels.
• Cate Brinson, Northwestern University: Polymer nanocomposites.
• Wes Burghardt, Northwestern University: Rheology of polymer gels.
• Costantino Creton, ESPCI (Paris, France): Adhesion of soft materials.
• David Dunand, Northwestern University: Thermoreversible gelcasting of titanium.
• Katherine Faber, Northwestern University: Thermoreversible gelcasting of ceramics.
• Dan Harrington, Northwestern University (Medical School): Biomedical applications of triblock hydrogels.
• Diethelm Johannsmann, Technische Universität Clausthal (Clausthal, Germany): Advanced uses of the quartz crystal microbalance.
• Joe Lenhart, Army Research Lab (Aberdeen, Maryland): Mechanical and fracture behavior of silicone gels.
• Phil Messersmith, Northwestern University: Adhesion of DOPA-containing hydrogels.
• Lonnie Shea, Northwestern University: Mechanical properties of tissue scaffolds.
• Jin Wang, Argonne National Laboratory. Synchrotron x-ray studies of polymer/metal nanocomposite systems and of polymer interfaces.